This invention relates to a continuous-motion line-array telecine.
FIG. 1 of the accompanying drawings diagrammatically shows the optical path of a continuous-motion line-array color telecine. FIG. 1 does not represent a particular known telecine but illustrates various features to be found in such telecines. A linear light source 10, which may use a mirror for reinforcement, is used to illuminate uniformly an area of cinematographic film which includes the line to be scanned, with the aid of a condenser system 20. The film is continuously moved at a uniform rate which provides the vertical scan during each film frame. The direction of film motion in the film plane 30 is downwards, as shown in FIG. 1. The arrow A in the film gate represents an upward-pointing arrow in the scene.
An object lens 40 is arranged to focus the film plane 30 onto the image plane 50. For a monochrome telecine a single linear array sensor would be placed on the optical axis at the point 60 with its length perpendicular to the plane of the paper. The light falling on each of the consecutive equispaced elements of the array is proportional to the light transmitted through consecutive equispaced elementary areas in a line on the film.
The details of reading out information from the linear array sensor will not be described in detail, as they will be known to those skilled in the art, but the specific requirements are as follows. During a film frame, the read out of consecutive lines occurs at equal intervals of time, the time interval being chosen so that the vertical spacing of the scanning lines is correct. This requires consideration of the speed of motion of the film, the width of the scanned area on the film, the required aspect ratio and the number of active lines in a frame. If film prints are intended for anamorphic projection the extent of the anamorphic squeeze also needs to be considered.
The film motion causes each film frame to be scanned once in the vertical direction, giving a progressive or non-interlaced line scan, so interlace is produced by writing the information sequentially into a frame store and reading alternate lines from the store on one field and the other lines from the store on the next field. Depending on the standards of the film in terms of film frames per second and the field frequency of the television system the number of field read-outs from the store varies but it is important to maintain the interlace of the signal read out.
It is usual for the ratio of (a) the desired height of the picture to be scanned in the active television lines, to (b) the spacing between like points of successive film frames, to differ from the ratio of the active field duration to the total field duration. This may be most elegantly overcome by starting the line scan operation at the beginning of each frame at the same point with respect to each film frame. However other solutions are possible but they are often only approximations.
The constant velocity of the film provides the motion which results in the vertical scan. However, it may be easier to think of this as providing a motion of the image in the image plane 50. The horizontal scanning of this image occurs in the linear array.
To provide a color telecine the light from the film has to be led to three linear array sensors for the red, green and blue color components, respectively. This could be done by using a splitter block containing dichroic optical filters to provide wavelength-dependent splitting. However, the need for a splitting block may be avoided by placing the three linear array sensors above one another in the image plane, as shown at 61, 62 and 63 respectively in FIG. 1. Each of the three linear array sensors has an optical filter 71, 72, 73 with an appropriate pass-band to make the individual sensors respond to essentially only to red, green and blue light, for sensors 61, 62 and 63 respectively. The order of sensors shown is essentially arbitrary. It should be noted that the figure is not to scale.
FIG. 1 also shows an optical partially-reflecting surface 56, the proportion of light being reflected to that transmitted being either constant or wavelength dependent, which is used to provide an optical path to a fourth linear array sensor 64, which may be a luminance sensor. This sensor is so positioned that it is observing the same line on the film as sensor 62. The number of linear array sensors using the reflecting path need not be limited to one. Indeed there is no absolute limit to the number of sensors using either the direct or the reflecting path.
It will be noted that a single line on the film will first be imaged on the sensor 63. Then, after the film has travelled a short distance, this line on the film will be imaged on sensor 62. Again, after a further short time interval, while the film travels a further short distance, the same line on the film will be imaged on the sensor 61.
At any one instant, the three sensors 61, 62, 63 are responding to light passing through the film along three different lines across the film. It is of course necessary that the light source 10 illuminate all the lines to which, at any time, the sensors are responding. The condenser system is designed with this in mind.
Without correction, the separation between those lines on the film that the sensors are responding to would cause severe vertical misregistration of colors. This misregistration may be decreased by reducing the separation between the sensors, but it can not be completely removed and it is still necessary to compensate for the remaining error. The compensation is achieved by delaying two of the signals by an amount equal in each case to the time by which they would otherwise be advanced with respect to the third signal. These time delays will depend on the velocity of the film, and it is also necessary to arrange that the timing of the start of the line scans of each advanced sensor is such that, when the signal has been delayed to remove the vertical misregistration, the timing of the start of the line of the delayed signal must be the same as that of the signal from the sensor which needs no delay.
The arrangements for this are diagrammatically illustrated in FIG. 1. The output of sensor 61 is applied directly to an output 81 for the R signal. The output of sensor 62 is applied to an output 82 for the G signal through a delay 84 providing a delay of time t. The output of sensor 63 is applied to an output 83 for the B signal through a delay 85 providing a delay of time 2t. Corresponding adjustment has to be made to the control signals used to cause the sensors to start to output a new line. Line scan control circuit 90 produces the necessary scan-control signals. As illustrated, read-out clock signals commanding the sensors to start to output a new line are applied directly to sensor 63, through a delay 91 providing a delay of time t to sensor 62, and through a delay 92 providing a delay of time 2t to sensor 61. The delays 91 and 92 are illustrative only and in practice their functions will be subsumed into the circuit 90.
The need to have different timing for the start of the line scans for the three sensors is disadvantageous, but the separation of the sensors may be varied to alter the required delay. This is used to make the required delays equal to multiples of the line scanning period; so that the same timing can be used for the start of line scanning for all three sensors. For systems where there is a disturbance to the line scan during the vertical blanking period there is a potential fundamental problem with this solution; this will arise if the longest signal delay were greater than the time between the end of scanning the active part of one frame of the film and the beginning of scanning the active part of the next frame.
The same line scan timing for all the sensors has certain benefits. It may make it possible to incorporate all three sensors in one device, leading to significant further benefits. For example, the sensors are likely to have more similar characteristics, reducing the probability of undesired color shading across the width of the picture. Other advantages accrue, for example the sensors can be brought closer to each other, so decreasing the amount of compensating delay.
However, there is the disadvantage that the spacing between the sensors must correspond to an integral number of picture lines on the image plane. This causes problems when the telecine is to be designed to produce television signals to different television standards. The picture line separation depends on the number of active lines of the television standard and on the effective height of the image of the film frame to be scanned. If the picture width is assumed to be constant and the spacing between sensors be 8 lines on a 1250 line 16:9 aspect ratio picture, this spacing would correspond to 6 lines for a 4:3 aspect ratio picture. The same spacing would correspond to 4 lines on a 625 line 16:9 standard and 3 lines if the aspect ratio be 4:3.
However the same spacing would not correspond to an integral number of lines on a 1050 line or 525 line standard with either 16:9 or 4:3 aspect ratios. A different spacing would be required, and yet another would be required for 1125 line 16:9 aspect ratio. Similar problems arise when different film formats are to be scanned.
Sometimes it is convenient to arrange a plurality of sensors with a lower number of elements to behave like a linear array of greater length. These sensors may look at different lines or parts of lines on the film image and the resulting signals combined by means including delays. The sensors may look at different parts of a line but a number of alternative approaches are possible. These include the one in which the sensors respond to every nth column in a picture. In this case, if there were 3 sensors, one would respond to elements in columns 1, 4, 7 . . . , the second to elements in columns 2, 5, 8 . . . and the third to elements in columns 3, 6, 9 etc.
In another arrangement, four line array sensors may be used to generate one scanning line. This may be done so as to provide a high resolution along the line scan; if a single array is used for this purpose impossibly high clock speeds are required to empty the array. The four arrays are arranged in a so-called castellated form, with the first and third arrays at one vertical position and the second and fourth arrays at a slightly different vertical position. This avoids any discontinuity at the transition points. Appropriate electronic delays are of course required. However, it will be apparent that there is again a situation where arrays which are supposed to relate to a single scanning line are spaced apart in the direction of film movement. The same problem as just discussed for a color device will thus apply for this monochrome device if it is attempted to accommodate multiple standards or formats.
An important case would be the use of this castellated approach providing improved definition for the luminance component in combination with the more normal approach for the color components where definition is, by comparison, less important.